The CAA said incident clusters around greater London and the home counties were a symptom of crowded skies. However, other clusters – such as those over East Anglia, Yorkshire and Lincolnshire – were caused by largely empty skies.

“Pilots in this more open air space tend to relax and think they are in a much freer, more open, sky,” said CAA spokesman Jonathan Nicholson. And that, he says, “can lead to an accident or an airprox”.

Image caption Col Donn Yates says many people do not appreciate the sheer size of an F-15

Hundreds of the more than 2,000 incidents investigated by the UK Airprox Board in the past decade involved military aircraft, including planes, helicopters, gliders, drones and even parachutists.

Fighters at Suffolk’s RAF Lakenheath have been involved in nearly 30 airproxes. Recent examples include two F15s coming within 500ft of a civilian B350 aircraft over Marham in Norfolk and an F15 passing close to a police drone at about 500mph (800km/h) on the edge of Dartmoor.

But Colonel Donn Yates, who until July this year was responsible two squadrons of combat ready F-15s at Lakenheath, said he felt some airproxes were not as close as thought.

He said even well-trained civilian pilots tended to underestimate the full size of an F15 fighter jet.

“So when we see in reports people saying this person was 300ft or 500ft away in actuality we find the person was actually a mile or a mile-and-a-half away,” he said.

Not all accidents or near-misses looked at by the BBC happened over land. Many took place at sea.

In 2013, a Super Puma helicopter ditched into the North Sea off Shetland with 18 workers from the Borgsten Dolphin platform aboard. Four people died.

In the past three years the AAIB investigated six commercial helicopter accidents out on North Sea platforms – three of them in 2017 alone.

Image copyrightPhil Breeze-LambImage caption Phil Breeze-Lamb compares flying a helicopter in poor weather with trying to “balance a marble on an upturned bowl”

Flying a helicopter in foul weather, says pilot Phil Breeze Lamb, is like “trying to balance a marble on top of an upturned bowl”.

“Off-shore is an obstacle rich environment,” he said. “On a good day it can be a piece of cake. In bad weather, it is completely different.”

Flying instructor Paul Bazire, based at North Weald in Essex, has been involved in two airprox incidents and seen first-hand the result of a fatal accident that killed two men in 2014.

He said the relatively high number of accidents and near-misses in the South East reflected the high level of activity there.

Image caption Paul Bazire has pilots from Poland and the Czech Republic coming to his flying school to learn how to deal with the busy skies of the South East

“The skies in the South East are much more compressed than elsewhere,” he said.

Some of his customers come from as far away as Poland to fly in the South East precisely because the skies there are so busy.

“It is said that if you can fly here, you can fly anywhere,” Mr Bazire said.

“Aviation in the UK is really safe,” said Mr Nicholson of the CAA. “It is 20 times safer to fly in a light aircraft than to ride a horse.”

Image caption Jonathan Nicholson of the Civil Aviation Authority says he hopes pilots will fit small transceivers to their aircraft to improve safety

A decade ago, the CAA tried to get private pilots to fit their aircraft with transponders – devices that identify a craft to air traffic control. But the proposals met with strong resistance, he said, because of the costs pilots would have incurred fitting the devices.